Biowaste-derived electrode and electrolyte materials for flexible supercapacitors

One of the key challenges in the development of energy storage devices relates to material sourcing in harmony with clean technologies. Herein, cellulose nanocrystals (CNC) extracted from brewery residues are used as transparent hydrogel electrolyte after physical cross-linking with aluminum ions (Al³⁺). The hydrogel electrolyte (Al-CNC) exhibits an ultrahigh ionic conductivity (∼24.9 mS cm⁻¹), high optical transmittance (∼92.9% at 550 nm wavelength), outstanding compression strength (3.9 MPa at a 70% strain), and tolerates to various deformations (e.g., twisting, folding, rolling). Meanwhile, animal bone biowaste is used to synthesize porous carbon (PC) electrodes (∼879 m² g⁻¹) that are effective in delivering an outstanding specific capacitance (∼804 F g⁻¹ at 1 A g⁻¹). A fully renewable flexible symmetric supercapacitor is assembled by sandwiching the Al-CNC hydrogel between two bone-derived PC electrodes (PC//Al-CNC//PC). The obtained flexible device displays a high energy density (18.2 Wh kg⁻¹ at 1 425 W kg⁻¹), exceptional power density (20 833 W kg⁻¹ at 7.1 Wh kg⁻¹), and ∼92% capacitance retention after 6 000 cycles at 5 A g⁻¹. We further demonstrated the biowaste-derived high-performance flexible supercapacitors for their mechanical durability and reliable electrochemical performance under bending cycles. All combined, the devices are shown to be ideally suited for renewable energy storage applications.